System topologies for optimum capacity transmission over wireless local area networks

ABSTRACT

An inventive method provides optimum topology for a multi-antenna system dedicated to higher throughput/capacity by bundling the Point Coordination Function (PCF) operation in infrastructure mode of the current and/or enhanced IEEE MAC with PHY specifications that employ some form of coherent weighting based on CSI at the transmitter in conjunction with the corresponding optimum receiver detection based on CSI. Specifically, CSI is measured from a control message, so data messages and control messages are separated. In the contention period of IEEE 802.11, the RTS/CTS exchange is used for CSI and the data message is sent following the CTS message. In the contention free period, a poll by the PC is separated from a data frame, which gives the polled station the first opportunity to send a data message. This change in topology results in various changes to the frame exchange format in the CFP for various scenarios of data and control messages to be exchanged.

PRIORITY STATEMENT

The present invention claims priority from co-pending Provisional U.S.Patent Application No. 60/460,553, filed with the U.S. Patent Office onApr. 4, 2003.

FIELD OF THE INVENTION

The present invention relates broadly to Wireless Local Area Networks(WLANs) and specifically to a topology for multi-channel wireless timedivision duplex (TDD) systems so that channel state information (CSI)may be acquired and used to optimize data throughput.

BACKGROUND

Highly functional computers have been interconnected with one another inwhat is termed a local area network (LAN) to enable users of individualcomputers within a predefined set to share files with one another.Traditional hardwired LANs are being superceded by wireless LANs (WLANs)as WLANs realize increased capacity. Data protocols for WLANs aregenerally organized into layers or levels of the communication system,each layer facilitating interoperability between various entities withinthe network.

The Institute of Electrical and Electronic Engineers (IEEE) standard forWLANs, IEEE 802.11, provides protocols for a physical (PHY) layer and aMedium Access Control (MAC) layer, shown in block diagram form at FIG.1A. The following discussion relates to that 802.11 standard in itscurrent form, though it is evolving. The PHY layer 21 provides protocolfor the hardware of WLANs termed stations or nodes. A station may bemobile station, wireless enabled laptop or desktop personal computer,and the like. The PHY layer concerns transmission of data between thosestations, and there are currently four different types of PHY layers:direct sequence spread spectrum (DSSS) 22, frequency-hopping spreadspectrum (FHSS) 23, infrared (IR) pulse modulation 24, and orthogonalfrequency-division multiplexing (OFDM).

The MAC layer 25 is a set of protocols that maintain order in the use ofthe shared bandwidth or medium, and the 802.11 standard specifies twomodes of communication: a compulsory Distributed Coordination Function(DCF) 26, and an optional Point Coordination Function (PCF) 27. A BasicService Set (BSS) 31 is shown in FIG. 1B, and is defined as a group ofstations 32 that are under the control of a single coordinationfunction, which in 802.11 is termed a Point Coordinator PC that may alsobe an Access Point (AP) 33. A BSS is roughly analogous to a group ofmobile telephone users within a cell of a single base station, with thebase station as the AP 33. Conceptually, every station in a BSS cancommunicate with every other station in that BSS, though degradations tothe transmission medium due to multipath fading or interference fromnearby BSSs can result in ‘hidden’ stations. The 802.11 standardprovides for two types of networks: ad hoc and infrastructure.Individual stations in the ad-hoc network are deliberately grouped as aBSS, but any station in the BSS may communicate directly with any otherstation in the BSS without channeling all traffic through thecentralized Access Point (AP). A good example of an ad hoc network is ameeting where employees bring laptop computers together to communicateand share files. One of the stations serve as a Point Coordinator tocoordinate transmissions and avoid collisions, but the PC in an ad hocnetwork does not act as an AP 33 that may link the BSS 31 to other BSSsor networks. Conversely, the infrastructure network uses one or morefixed network APs 33 by which wireless stations can communicate beyondthe BSS 31. These network APs are sometime used to bridge the BSS toother BSSs to form an extended service set (ESS) and/or to wirednetworks such as the internet or a conventional intranet as shown inFIG. 2A. If AP service areas overlap, handoffs can occur for roamingstations that move between APs similar to cellular networks commonlyused for mobile telephony. In the MAC layer, the DCF operates in both adhoc networks and infrastructure networks. However, since PCF requires anAP 33, PCF may operate only in infrastructure networks.

Avoiding collisions (simultaneous transmissions) between stations in aBSS is complicated by the fact that while a wireless station istransmitting, it cannot monitor the transmission medium (the channel orchannels) for other traffic that may interfere with its owntransmissions. For example, one problem arising from the inability tolisten while transmitting in WLANs is termed a “hidden node”. Assumestations A, B and C in a BSS are disposed as in FIG. 1B, with Bphysically located between A and C. If stations A and C cannotcommunicate directly with one another due to distance, multipath fading,or some other reason, stations A and C are hidden from one another.Absent some collision control scheme, station A may listen to thechannel, sense it is clear, and transmit a packet to station B. Whetheror not station C is transmitting to B is unknown to A, except throughcoordination by the PC. Simultaneous transmissions from stations A and Cto station B would result in collision and lost transmissions, since allstations in a BSS 31 communicate over the same channel.

DCF seeks to minimize collisions by prioritizing stations waiting totransmit based on a time delay basis. In DCF, each station 32 with adata message to transmit contends for the next available slot on the BSSchannel during what is termed a contention period CP 29. Time delays forvarious stations have a random component, but procedures ensure awaiting station moves up in priority the longer it waits. Details of theDCF prioritization protocol are described in detail below. Once astation sends its data message, which is included in a MAC Service DataUnit (MDSU), it must contend with all other waiting stations for anotheravailable slot. PCF is provided to avoid the situation wheretime-sensitive data from one station cannot be assembled into one MDSU,which is constrained to a maximum length. For example, station A maywish to send an audio or video clip that spans three MDSU's to stationB, but contending for a separate transmission slot for each of the MDSUswould potentially result in the clip being undecipherable. While arelatively large buffer in the receiving station may store andre-assemble the separately received clip portions after a notinsignificant delay, that option is generally not seen as viable in thelong term due to the dual constraints of low power consumption and smallphysical size of wireless stations. When implemented, PCF takes priorityover DCF in that a contention free period (CFP) 28 is establishedwhereby station A may send its data messages without contending for atime slot. During the CFP 28, other stations stand by and await either apoll by the PC during the CFP 28 or a contention period (CP) 29 in whichthe various stations contend for a slot as in DCF above. Additionaldetails of PCF are provided below.

Historically, the development of WLAN systems, and wireless systems ingeneral, have taken two paths, one focused on specifications for the PHYlayer and the other for the MAC layer. For example, the IEEE 802.11(e)task group is developing MAC layer enhancement to improve MAC layerthroughput regardless of physical layer throughput. The IEEE 802.11(g)task group has developed a physical layer specification that facilitatedata rates of 20+ megabits per second (Mbps) in the 2.4 GHz. Range, butmust keep MAC layer changes to a minimal. Though both working groupsoperate concurrently, in practice there appears little interactionbetween the two groups. Advantages that may be gained by a more holisticapproach are never recognized by the groups' single-layer focus.

Recently, the IEEE has approved a High Throughput Study Group (HTSG) for802.11, whose charter is to provide higher throughput than enabled bycurrent IEEE 802.11 standards. The High Throughout Task Group (HTTG)will develop the actual standards, which appears to be the first timethat modifications to the MAC and physical layers will be developedcoherently since the division of those layers. A recent study showedthat the current IEEE MAC and physical layers is limited to a throughputof 0.2 Mbps per 1000 byte packet per operational mode. Existing 54 Mbpsmodes therefore have approximately 28 Mbps throughput for a 1000 bytepacket. Maintaining the same ratios, then a 108 Mbps data rate yields athroughput of 56 Mbps for a 1000 byte packet.

It is well-known that optimum capacity is achieved when Channel StateInformation (CSI) is known and used at both the transmitter andreceiver, and that MIMO systems (multiple input/receive antennas and/ormultiple output/transmit antennas) provide a substantial increase incapacity as compared to more traditional systems employing a singleantenna on all transceivers. For example, knowing CSI enables atransmitter to parse data among different channels in a manner thattakes advantage of the entire channel capacity on each channel, ratherthan allowing the time-sensitive bandwidth to be not fully used. Somecommunication standards such as Code Division Multiple Access (CDMA)reserve a feedback channel to provide CSI to the transmitter.Unfortunately, CSI via a feedback channel is imperfect due to feedbackdelays and changing channel characteristics. Regardless, the 802.11standard does not entail a feedback channel, there are no physical layerspecifications in 802.11 that are based on CSI, and some researchersbelieve the lack of CSI in the standard prohibits the adoption of afeedback channel in future versions of 802.11.

Thus, there is a need in the art to provide an optimumthroughput/capacity topology for multi-antenna wireless systems thatimposes changes that are backwards compatible with current WLANstations.

SUMMARY OF THE INVENTION

Fortunately, there are resolutions to this problem that are embodied inthe present invention. As mention above, there are no physical layerspecifications in the IEEE 802.11 standard that are based on CSI at thetransmitter. Operation of the Contention Free Period (CFP) is describedin the IEEE 802.11(e) draft standard, herein incorporated by reference.Depending on the physical layer standard 802.11(a), 802.11(b) or802.11(g), the CFP modulation is derived from one of their operationalmodes.

A system according to an embodiment of this invention provide theoptimum topology for a multi-antenna system dedicated to higherthroughput/capacity by bundling the Point Coordination Function (PCF)operation in infrastructure mode of the current and/or enhanced IEEE MACwith PHY specifications that employ some form of coherent weightingbased on CSI at the transmitter in conjunction with the correspondingoptimum receiver detection based on CSI.

In one embodiment of the present invention is a method of communicatingover multiple sub-channels of a WLAN. The method includes sending acontrol message that is not combined with a data message from a firstnetwork entity to a second network entity. The control message may be,for example, a CTS message during the CP or a poll during the CFP, butin any case the control message is to facilitate sequencing of wirelesstransmissions among at least two entities in a wireless network. In theinventive method, the control message is received at the second networkentity, which uses it to obtain channel state information CSI. The CSIis used to determine the capacities of at least a first and secondsub-channel of the wireless network, and to determine which has thegreater capacity. A data message to be sent is divided into at least afirst and second data message segment, wherein the relative sizes of thesegments are based on the relative capacities of the sub-channels. Thedivision itself is preferably via an eigenmode or water-filling known inthe art to exploit varying capacities of sub-channels. When the firstsub-channel is determined to have the greater capacity, the first datamessage segment will then define a greater size than the second datamessage segment. Further in the method and in response to receiving thecontrol message, the second network entity sends the first data messagesegment over the first sub-channel, and the second data message segmentover the second sub-channel of the wireless network. In this manner, CSIis obtained and used to send the segmented data message, though notnecessarily the control messages.

In a particular embodiment, the first network entity is a pointcoordinator PC of a wireless network basic service set BSS operatingduring a contention free period CFP, the control message is a poll ofthe second network entity, and the PC may respond with an ACK messagecombined with a data message for the first network entity. Preferably,where the PC sends a poll of a third network entity during the same CFPas the poll of the second network entity, and the PC fails to receive aresponse from the third network entity within a first time period suchas a SIFS, the PC then polls a fourth network entity within a secondtime period such as a PIFS that is no greater than twice the first timeperiod. Where the PC receives from a network entity an ACK messagecombined with a data message, the PC may respond with an ACK messagecombined with a separate control message that signals an end of acontention free period. In the 802.11 standard, for example, such amessage from the PC would be a combined ACK and CFP-End message.

Further according to another aspect of the present invention, when themethod is executed during a contention free period CFP, and the firstnetwork entity is a point coordinator PC and the control message is afirst poll of the second network entity, there exists an instance wherea polled station does not respond to its poll. To avoid confusion withthe terms above, assume an initial poll of an initial network entity orstation occurs prior to the poll of the second network entity orstation. Prior to sending a control message without a data portion fromthe PC to the second network entity, the method preferably also includessending from the PC an initial poll without a data message to an initialnetwork entity. Upon the PC failing to receive a response to the initialpoll from the initial network entity within a first time period such asa SIFS, the PC then preferably sends, within a second time period suchas a PIFS that is greater than the first time period, either a datamessage to the initial network entity or the first poll of the secondnetwork entity as described above.

The present invention may also be adapted for station-to-station datacommunications during the CFP. Where the method as summarized above isexecuted during a CFP, the data message in its various segments is sentover the sub-channels from the second network entity to a third networkentity that is not a point controller PC. In that instance, the methodfurther includes the third network entity sending to the second networkentity an ACK message within a first time period, in response toreceiving the data message segments. The PC may then send, within aperiod of time following the ACK message from the third entity that isless than twice the first time period, either a poll to a networkentity, or a data message to the second network entity that is dividedinto data message segments based on CSI that is measured from at leastone data message segment sent from the second network entity to thethird network entity. If the PC is to allow the second and thirdstations to exchange multiple data messages between them, the PC willwait a PIFS before transmitting. If the PC is to allow only one cohesivedata message from the second to the third entity, it need wait only oneSIFS after the ACK message from the third to the second entity, or onePIFS following the data message from the second to the third entity.

In the above method, at least one of the network entities is preferablya mobile station such as a mobile phone. The term mobile station as usedherein includes any portable electronic device that has a telephoniccapability, such as cellular phones, portable communicators, PDAs withtelephonic capability, and further includes the various accessories tothe above that expand the capabilities or functionality of the mobilestation with which they are coupled.

According to another embodiment of the present invention is a method ofcommunicating data over a wireless network according to an IEEE 802.11standard as it exists as of the priority date of this application. Inthis embodiment, the improvement to the 802.11 standard includesseparating by at least one Short InterFrame Space SIFS a poll and a datamessage sent by a point controller PC while in a contention free periodCFP. This allows data messages sent from the PC to be transmitted withthe benefit of knowing CSI, with at least one possible exception notedbelow.

Preferably, CSI is also obtained during the contention period CP duringa Request-to-Send/Clear-to-Send RTS/CTS exchange. In that instance, CSIis used to determine relative capacities of at least a first and secondsub-channel to parse a data message from a station sending the RTS to astation sending the CTS. Specifically, a data message from theRTS-sending station is parsed into at least a first data message segmentdefining a first size and a second data message segment defining asmaller second size. The relative segment sizes are based on relativecapacities of a first and second sub-channel as determined by themeasured CSI. The larger first data message segment is sent over thehigher capacity first sub-channel and the smaller second data messagesegment is sent over the lower capacity second sub-channel. Parsing ofthe overall data message is based on relative sub-channel capacity asdetermined by the measured CSI, such as by eigenmode or water-fillingtechniques known in the art.

Considering again the CFP, this embodiment of the present inventionpreferably restricts the PC to sending only one of five possiblemessages: a poll; a data message parsed according to measured CSI andtransmitted among at least two sub-channels; a data message so parsedand transmitted combined with an ACK message; a CFP-End message; and aCFP-End message combined with an ACK message. Conversely, 802.11currently allows a data message to be combined with a poll message, anddoes not provide that an ACK can be combined with a CFP-End messagesince there appears no opportunity for the latter to ever need becombined as the standard currently exists. Preferably, the PC cancombine a data message only with an ACK message, else the data messagemay not be combined with any other message.

Preferably, the PC is allowed to send a data message without validmeasured CSI to a station only upon non-receipt of a response from thatsame network entity to its poll within one SIFS. Most preferably, the PCcan only send a data message with either valid measured CSI or estimatedCSI.

Where the PC and the polled station each have a data message to send,one difference of the present invention as compared to the 802.11standard is that the polled station is preferably allowed to send itsdata message first. Preferably, between the time the PC polls thestation and the time the PC may next transmit, the polled station maysend a data message to another station (that is not the PC) withoutusing measured valid CSI for the channel between the polled station andthe another station. In this instance, the another station is allowed anopportunity (one SIFS) to send an ACK message to the polled stationprior to the time the PC is next allowed to transmit.

Another aspect of the present invention is a network entity forcommunicating over a wireless local area network, such as a mobilestation, a point controller, an access point, or any other entity on theWLAN. The network entity includes a receiver for receiving over at leasttwo sub-channels a control message from an entity of a wireless localarea network. The control message is preferably a CTS message or a poll.The mobile station further has a processor for determining a capacity ofa first sub-channel and a capacity of a second sub-channel based onchannel state information CSI measured from the control message. Itfurther includes means for parsing a data message into at least firstand second segments based on the relative determined capacities of thefirst and second sub-channels. To best exploit the multi-channelcapability in both transmit and receive functions, the mobile stationhas a first and second antenna having inputs coupled to an output of themeans for parsing. The first antenna is for transmitting at least thefirst segment over the first sub-channel and the second antenna fortransmitting at least the second segment over the second sub-channel. Incertain embodiments, there may be a crossfeed between antennas withdifferential weighting for each data message segment so that eachsegment is actually transmitted over each sub-channel, and increasedcapacity is realized by the differential weights assigned to eachsegment.

These and other features, aspects, and advantages of embodiments of thepresent invention will become apparent with reference to the followingdescription in conjunction with the accompanying drawings. It is to beunderstood, however, that the drawings are designed solely for thepurposes of illustration and not as a definition of the limits of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is better understood in light of the followingdrawings.

FIG. 1A is a prior art block diagram showing MAC and PHY layerstructures in 802.11.

FIG. 1B is a prior art block diagram showing BSS's connected to a wirednetwork by a Distributions System.

FIG. 2A is a prior art timing diagram showing a CFP overlain on aregular system implementing pure DCF.

FIG. 2B is similar to FIG. 2A but reflecting changes according to thepresent invention.

FIG. 2C is similar to FIG. 2B but showing a different exchange of datapackets.

FIG. 2D is similar to FIG. 2B but showing yet another different exchangeof data packets.

FIG. 3 is a timing diagram showing a RTS/CTS Frame Exchange during thecontention period.

FIG. 4 is a prior art block diagram showing a fragmentation in IEEE802.11 MAC.

FIG. 5 is a prior art block diagram showing a IEEE 802.11 data frameformat.

FIG. 6 is a prior art block diagram showing an ACK frame.

FIG. 7 is a prior art block diagram showing a PS-Poll Control Frame.

FIG. 8 is a prior art block diagram showing a RTS Control Frame.

FIG. 9 is a prior art block diagram showing a CTS and ACK Control Frame.

FIG. 10 is a graph of 2×2 Capacity curves m Rayleigh channels.

FIG. 11 is a PDSU for Optimum Topology according to the presentinvention

DETAILED DESCRIPTION

In the 802.11 standard, a Point Controller (PC) coordinatesprioritization during the contention free period CFP 28. The PC isfunctionally within the Access Point (AP) 33 of a BSS 31 and is usuallyphysically collocated with it, so the term AP 33 is used herein toindicate either or both the AP 33 and PC. A station 32 may serve as theAP 33 and the CP. FIG. 2A is a prior art timing diagram showingtransmissions sent (above the line designated 34) and received (belowthe line 34) by the PC according to the 802.11 standard. The time periodillustrated in divided into the contention free period 28 and thecontention period 29, which together comprise a CFP Repetition interval35 sometimes referred to as a superframe. The CFP repetition intervals35 continue so that, when PCF 27 is in use, the CFPs 28 and CPs 29alternate. The CFP is described with reference to FIG. 2A, and the CP isdescribed below in conjunction with the distributed coordinationfunction DCF 26. Prioritization of transmissions by the various stations32 in a BSS 31 is therefore via PCF 27 during a contention free period28, and via DCF 26 during the contention periods 29.

A superframe 35 begins with a beacon frame 36 transmitted by the PC,regardless of whether PCF is active or not. The beacon frame 36 is amanagement frame that provides timing and protocol related parameters tothe stations. Each beacon frame 36 also announces when the next beaconframe will be transmitted, so that all stations 32 are aware ofsuperframe lengths. To enable PCF 27 to take priority over DCF 26, thePC transmits the beacon frame 36 after a PCF Interframe Space (PIFS) 37(about 25 μs) following the end of the last superframe 35. Because thePIFS 37 is shorter than a DCF Interframe Space (DIFS, about 34 μs) thatthe DCF 26 must wait following the end of a superframe 35, PCF 27 cantake priority. A Short Interframe Spacing (SIFS) 38 spans about 16 μsand is the amount of time a station 32 is allowed to reply to the PC.Each station 32 within the BSS 31 resets a Network Allocation Vector(NAV) 41 based on the beacon frame 36. In FIG. 2A, the NAV 41 informsthe station 32 to set the beginning of the next CP 29 at the maximumspan, and not to transmit during the intervening CFP 28 except under twocircumstances: in response to being polled by the PC, or to send an ACKmessage within one SIFS after receiving a data message.

After the beacon frame 36, the PC delays one SIFS 38 and may send any ofthe following: a data-only frame, a data+poll frame 42, a poll-onlyframe, or a CFP-end frame. The PC maintains a list of stations for whichit has data, and typically polls those stations first in order topiggyback that data with its poll of the station. Referring to FIG. 2A,the PC polls a first station and piggybacks data with that poll in adata+polling frame 42 (both data and poll are directed to the firststation). Upon receiving the data, the first station responds with anacknowledgement (ACK), but itself piggybacks data (U1) on its ACK in adata+ACK frame 43. The first station is allowed a SIFS 38 to respond tothe AP's poll, but may send its data (U1) to any station or to the PC.[If it is sent to a station other than the PC, that station has one SIFSto send its ACK, without piggybacking data, back to the first station.]

After receiving the data+ACK frame 43 from the first station (U1+ACK),the PC waits one SIFS and polls another station (or the same station).In the event the previous first station sent its data (U1) to the PC,the PC will piggyback an ACK for that first station in the data+poll itsends to a second station in a data+poll+ACK frame 44 (D2+ACK+Poll, dataand poll directed to the second station, ACK directed to first station).In FIG. 2A, the second station does not respond within one SIFS, soafter waiting a total of one PIFS, the PC sends a poll with data (D3) toa third station in another data+poll frame 42 (D3+Poll, data and poll tothird station). The third station responds within a SIFS with data (U3)and an ACK in its data+ACK frame 43. When the PC has no more stations topoll, or when the CFP as determined by the beacon frame 36 nears itsend, the PC transmits a CFP-End frame 45 to signal all stations 32 thatthe CFP 28 is ended.

One drawback with the prior art, at least in certain circumstances, isthat the polling frames and the data frames from the PC may be combinedinto a single frame (data+poll 42 or data+ACK+poll 44). At the time ofthat combined frame transmission, the PC does not know the channel statebetween it and the intended station. While channel state may not changesignificantly over a single CFP repetition interval 35 when used in awired network, channel states change much more rapidly in WLANs. Toincrease capacity over a fixed bandwidth, multiple sub-channels arepreferably used such as in a single input/multiple output (SIMO)communication system, a multiple input/single output (MISO) system, ormost preferably a multiple input/multiple output (MIMO) system. Any ofthese are referred to hereafter as a MIMO system unless otherwisestipulated. The multiple sub-channels of a wireless MIMO system are eachsubject to rapid changes due to fading, multipath, etc., so wirelessMIMO systems need to know the state of the different sub-channels tosend different data portions over the strongest channels, or topartition the data to be sent into sizes that maximize the respectivecapacities of the various sub-channels as those sub-channels exist atthe time of transmission. When the PC polls a station, it has not yetreceived any feedback from that station by which to measure the truechannel. Since the sub-channels change rapidly, it is highly unlikelythat the coherence interval (the interval over which the measured stateof the channel does not change significantly) spans an entire CFPrepetition interval 35. Said another way, any measurements of thechannel made in one CFP 28 are unlikely to be valid estimates of thechannel during the next CFP 28. Sending a data message combined with apoll necessarily implies sending the data either regardless of channelquality or with invalid (i.e., outside the coherence interval) estimatesof the channel. Either option is a waste of bandwidth as compared tomaximum capacity theory. Among other aspects, the present inventionmodifies the specific frame exchange of FIG. 2A to enable entitiestransmitting data frames to do so with knowledge of the channel, termedin the art as channel state information or CSI.

FIG. 2B is similar to FIG. 2A but shows the same substantive exchange ofdata frames depicted in FIG. 2A (one data frame from the AP to each of afirst, second, and third station, and one data frame from the first andthird stations to the AP) accomplished according to the presentinvention. For each of FIGS. 2B-2D, only the CFP 28 is shown and theinterval between frames is one SIFS unless otherwise noted. At the startof the CFP 28, the PC transmits a beacon frame 36 as described. The PCnext transmits a polling-only frame 46 (P1) that is directed to thefirst station. The first station has a data frame for the PC, and hasthe opportunity to measure actual CSI between it and the PC in thepolling frame 46. The first station uses that CSI to send a data onlyframe 47 back to the PC within one SIFS of the end of the polling frame46. The PC receives the data only frame 47 (designated U1) and uses itto measure the channel between it and the first station. Using that CSI,the PC then sends its data for the first station combined with anacknowledgement that it (the PC) received the data frame from the firststation in a data+ACK frame 43. This obligates the first station toreply with an ACK only frame 48 that it received the data correctly.After a SIFS, the PC then polls the second station (P2) in apolling-only frame 46. The second station does not respond within aSIFS, so after a total delay of one PIFS, the PC polls a third station.The exchange between the PC and the third station is similar to thatdescribed between the PC and the first station for FIG. 2B.

On first glance, it appears the exchange of frames of FIG. 2B introducesan inefficiency as compared to that of FIG. 2A, due to an increasednumber of frames and interframe spacings. However, the poll only 46 andACK only 48 frames are quite short, whereas any frame that includes data42, 43, 44, 47 may be substantially longer. In the present invention asembodied in FIG. 2B, the poll only frames 46 may be sent without validCSI and all frames that include data are transmitted to maximize theavailable capacity of the channel. Preferably, all frames carrying dataare sent with valid CSI by use of the present invention, though FIG. 2Cnotes an exception. While additional MAC overhead may be increased ascompared to the method of 802.11, throughput is increased due to thelarger size of frames with data as compared to those without. Variousframe sizes and throughputs are detailed below with reference to FIGS.5-10.

FIG. 2C is an illustration of frame exchange for the instance where theAP has data for the first and third station, and only the third stationhas data for the PC. The beacon 36 and polling only 46 (P1) frames areas described with reference to FIG. 2B. Since the first station of FIG.2C has no data for the PC, it does not respond to the poll within a SIFSand the PC is allowed to transmit again after a PIFS 37. In oneembodiment of the invention, the PC sends a data-only frame 27 (D1) tothe first station without having had an opportunity to measure CSI(since the first station did not respond to the poll within a SIFS). Thefirst station sends an ACK only frame 48, and the remainder of FIG. 2Cis similar to FIG. 2B except the portion beginning with the framedesignated ACK+U3. Rather than sending an ACK only frame 48 as in FIG.2B, the third station has data for the PC, which it sends with anACKnowledgement in a data+ACK frame 43. Assuming there are no furtherstations for the PC to poll, it responds to this last transmission fromthe third station with an ACK+end frame 49, wherein the ACK is directedto the third station and the CF-END portion is directed to all stations32 of the BSS 31.

As an alternative to the scenario described for FIG. 2C wherein the PCsends a data only frame 47 to the first station without benefit of CSI,the first station (or any station being polled but not having data totransmit to the PC) may be obliged to reply with an ACK only frame 48 inorder that the PC may measure the channel. Since the PC may also nothave data for the station responding to a poll with an ACK only frame48, there is a potential to waste bandwidth that in the cumulativebecomes non-negligible. This wasting aspect may be minimized byincluding within the poll frame information that indicates whether ornot the PC has data to send to the polled station, which may be aslittle as a single bit (e.g., 0 indicates no data, 1 indicates data).The polled station may disregard that information if it has data to sendto the PC (as in FIG. 2B), allow a SIFS to expire without responding ifthe information indicates there is data (as in the exchange depicted inFIG. 2C between the PC and the first station), or respond with an ACKonly frame 48 if the information indicates there is data coming from thePC (as in the exchange depicted in FIG. 2D between the PC and the secondstation).

FIG. 2D depicts frame exchange for additional scenarios. The beacon 36and exchange between the PC and the first station are as in FIG. 2C.Upon polling a second station with a polling only frame 46 (P2), thesecond station responds with a data frame to another station 51 ratherthan to the PC. This station-to-station data frame 51 is sent withoutthe benefit of valid measured CSI, since there is no priorcommunication, within the coherence interval, from the recipient of thestation-to-station data frame 51 by which to measure the channel. Therecipient station then responds with an ACK only frame 48 directed backto the sending station. Though the data in frame 51 was directed towardanother station, the PC still listens and uses it to measure the channelbetween it and the second station. Following the ACK only frame 48directed back to the second station, the PC may send a data only frame47 to the second station without drawing a direct response from it. ThePC may wait a PIFS, to allow the second station an opportunity to sendadditional station-to-station data frames 51. The second station sendsan ACK only frame 48 back to the PC, which then polls a third stationwith a polling only frame 46. The third station in the scenario of FIG.2D has no data to transmit, so the PC waits a PIFS 37 and transmits aCF-END frame 45 to transition into the contention period 29.

In any of the above instances, any of the PC or stations may have morethan one frame with data to send. Due to the potential size of the dataframes and the speed with which the channel may vary over time (thelength of the coherence interval), it may be necessary in one instancethat the sender re-acquire CSI from the last transmission of theintended recipient, and in another instance it may have negligibleeffect on data throughput that the sender re-use the originally measuredCSI. So long as the frames in question are sent within the coherenceinterval established when CSI is measured, then CSI is considered validwhether or not is was measured based on a frame received immediatelypreceding the next frame to be sent.

The above description pertains to the CFP 28 wherein the PC controlswhich station in an infrastructure network may next transmit. Followingis a description as to how the present invention may be used within thecontention period 29 following the CFP 28. Since the CFP 28 exists onlywhile in the point coordination function 27, operation within the CP 29is within the base DCF 26 layer of MAC 25 and is detailed at FIG. 3.

DCF lies directly on the PHY layer 21 and is based on Carrier SenseMultiple Access with Collision Avoidance (CSMA/CA) protocol, becausewireless stations cannot listen for collisions while transmitting. Asknown in DCF, when a station has a frame with data to be transmitted, itfirst listens to ensure no other station is transmitting over theprescribed channel and transmits only if the channel is clear for a setperiod of idle time, termed a DCF-interframe space (DIFS) 38 that islonger than a PIFS. If the channel is busy, the station instead choosesa random “backoff factor” which determines a delay period 58 wait untilit is allowed to transmit its data. During periods in which the channelis clear, the transmitting station decrements its backoff counter toshorten the delay period 58 so a delayed station gradually gains ahigher priority to transmit. When the backoff counter reaches zero andthe channel is clear for the duration of a DIFS 38, the station maytransmit its frame with data. Since the probability that two stationswill choose the same backoff factor is small, collisions between dataframes from different stations are minimized.

When a particular station's backoff counter reaches zero and it sensesthe channel is clear for an entire DIFS 38, that station, termed thesource 52 or transmitting station, first sends out a short ready-to-send(RTS) frame 53 containing information on the length of the frame withdata to be transmitted. If the intended destination 54 to which the RTS53 is directed hears it, the receiving station 54 responds with a shortclear-to-send (CTS) frame 55. Only after this exchange does the source52 send its data frame 47 during the CP 29. When the destination 54receives the transmitted data frame 47 successfully (as determined in802.11 by a cyclic redundancy check CRC), the receiving station (or PC)transmits an acknowledgment (ACK) frame 48. This back-and-forth exchangeis necessary to avoid the “hidden node” problem previously explained. Ifthe receiving station 54 has a data frame 47 to send, it must contendfor a transmit slot as above and cannot piggyback data onto its ACKframe 48. During this process, other stations 56 defer transmissionaccess 57 until they sense the channel is clear for a DIFS plus theirbackoff factor.

The present invention exploits the RTS/CTS interchange to provide validCSI to at least the source 54 for use in transmitting the data frame 47.The benefits of the destination 54 using CSI obtained from the RTS/CTSexchange for use in transmitting the ACK only frame 48 are relativelyminor as that frame is small. Since each station is at differing timesboth a source 52 and a destination 54, the means to implement thepresent invention are already in place and can be used for the ACK onlyframe 48, even if its practical effect is merely to send an unparsed ACKframe 48 over the most robust of the available sub-channels.

There is another opportunity within the 802.11 standard by which astation may obtain valid CSI for the channel over which it intends totransmit. A listening station, such as the other station 56 of FIG. 3that is not a source 52 or destination 54 of a particular exchange, maytransmit a CTS message to itself in accordance with the standard toobtain CSI. That CSI may then be used, within the coherence interval inwhich it is valid, to reserve the channel and preserve a clear channelaccess CCA mechanism.

FIG. 4 is a prior art block diagram of a MAC Service Data Unit (MSDU)58, the term used to represent units of transmission in the MAC layer 25of the 802.11 standard. As noted above, different messages may be“piggybacked”, and the different fragments 59 of the MDSU 58 representthose different messages, which may each be independently addressed.Each fragment includes a leading MAC header 61, a trailer 62 thatincludes a cyclic redundancy check CRC, and a frame body 53 betweenthem. A single MDSU 58 may include more than one frames or fragments 59(as in data+ACK frame, ACK+poll frame, etc.), or only one frame orfragment 59 (as in the poll only frame, data only frame, etc.)

FIG. 5 shows a more detailed view of a data only frame 47 that may beone of the fragments 59 of an MDSU 58. The number of octets dedicated toeach portion of the frame 47 is listed directly below the block. Each ofFIGS. 5-9 are known in the art and consistent with the 802.11 standard,and are presented hereto demonstrate quantitative gains in using thepresent invention as compared to the current 02.11 standard. In the dataonly frame 47 of FIG. 5, the various portions of the header 61 usethirty octets, the trailer 62 uses four octets, and the body 63 carryingthe substantive data may extend to 2312 octets, depending upon theamount of data to be sent. By comparison, FIG. 6 represents an ACK onlyframe 48 with a sixteen octet header 61, a four octet trailer 62, and afour octet body 63. FIG. 7 represents a poll only frame 48 with asixteen octet header 61, a four octet trailer 62, and a zero octet body63. FIG. 8 represents a RTS Control Frame 53 having the same relativesizes as those of the poll only frame 48 of FIG. 7 but with differentheader fields. FIG. 9 represents a CTS Control Frame 55 having a tenoctet header 61, a four octet trailer 62, and a zero octet body 63.Using these relative frame sizes, one can calculate the data throughputsfor various scenarios to compare a wireless network using the topologyof the present invention to the topology currently stipulated in the802.11 standard. Those calculations as concerning the present inventionare presented below.

The minimum criteria for optimum transmission topology for wireless timedivision duplex TDD networks are:

-   -   1) valid CSI is present at the transmitter,    -   2) eigen-mode transmission is performed, and    -   3) the frame/packet is received by the intended recipient within        a period less than the coherent time of the channel.

To achieve the capacities possible with the present invention, thetransmitter should employ some weighting mechanism to assign frames,packets, fragments, or whatever division of the entire package to betransmitted to various sub-channels based on the measured quality ofthose sub-channels. Eigen-mode or waterfilling is one technique known inthe art to do so, described mathematically below. For ad hoc networksand infrastructure networks during the contention period, the RTS/CTSexchange may be used. During the contention free period, the revisedframe exchange described above may be employed to achieve valid CSI. Ineither case, the coherent weighting is done at the PHY layer 21, so thepresent invention modifies both the MAC and PHY layers. TABLE 1 HalfDuplex Frame Efficiency for 1500 byte packets using Optimum TopologyConfigurations @ MAC SAP 12 24 54 6 Mbps Mbps Mbps Mbps 100 Mbps 200Mbps CFP-Poll 95.35% 93.05% 88.75%  79.6%  68.7% 52.93% CP-  93.8% 90.9%  85.6% 74.74% 62.55%  46.2% RTS/CTS

Frame Efficiency as used in Table 1 is the time required to transmit theinformation portion of packet divided by the total on air time forpacket. Thus, the overall capacity is found by multiplying the frameefficiency by the capacity/throughput, which are shown in Table 2 below:TABLE 2 802.11 Capacity Requirements in bps using Optimum TopologyConfigurations @ MAC SAP 54 100 200 6 Mbps 12 Mbps 24 Mbps Mbps MbpsMbps CFP-Poll 0.52 1.07 2.25 5.65 12.13 31.5 CP-RTS/CTS 0.533 1.10 2.346.02 13.32 36.1

The capacity requirements are computed as raw data rate/12Msymbols/sec/Frame efficiency to yield the target throughput/capacity atthe MAC SAP layer. The theoretical best performance for these capacityrequirements can be read from FIG. 10 for a 2×2 configuration (2 inputantennas, 2 output antennas) in Rayleigh fading, or computed using theformula below for any arbitrary MIMO configurations$C = {{\log_{2}\left\lbrack {\det\left( {I_{M} + {\frac{\gamma}{N}{HH}^{\dagger}}} \right)} \right\rbrack}\quad\text{bps/}{Hz}}$

Eigen-mode transmission as noted above is described as follows. Let thesingular value decomposition of H be H=UΣV where U and V are unitarymatrices and Σ be a diagonal matrix With positive real values on thediagonal elements representing the singular values of the channel. Ifthe transmitted vector r is pre-multiplied by V in the transmitter andreceived vector is post multiplied by U^(H) in the receiver, i.e., VrU^(H)=V (Hx+n) U^(H)=Σx+m, where m=Vn*U^(H) and there is no noiseamplification and remains spatially white.

Because a single MAC layer must interface with disparate PHY layers, the802.11 standard uses an additional protocol layer termed the PhysicalLayer Convergence Protocol (PLCP) disposed between them that is defineddifferently for each transmission method. The PLCP adds a preamble and aheader (each of various sizes) to a PLCP Service Data Unit (PSDU), whichis the portion of the complete transmission frame (PPDU or PLCP ProtocolData Unit at the PHY layer) that carries the actual data to betransmitted between stations or between the point controller PC and astation. FIG. 11 is a block diagram showing a PSDU 65 for optimumtopology according to the present invention, with times and numbers ofbits tailored for compatibility with the 802.11 standard as it currentlyis written. The present invention enables the length of a guard interval66 a, 66 b to be selectable (to vary) based on the CSI. For certainchannels, the delay spread of the channel is shorter than other time andhence not necessary to keep a fixed cyclic prefix (CP) overhead.Further, if capacity achieving codings are used, such as low densityparity check codes (LDPC) or Turbo codes, then additional time isallocated at the end of the packet for iterative decoding, which is notcurrently available in current IEEE 802.11 standard or its amendments.This additional time is represented in the PSDU 65 of FIG. 11 as aniterative decoding signal extension 67.

While there has been illustrated and described what is at presentconsidered to be a preferred embodiment of the claimed invention, itwill be appreciated that numerous changes and modifications are likelyto occur to those skilled in the art. It is intended in the appendedclaims to cover all those changes and modifications that fall within thespirit and scope of the claimed invention.

1-24. (canceled)
 25. In a wireless local area network wherein a firstnetwork entity transmits to a second network entity a packet having aguard interval preceding one of a data signal and a training sequence,the improvement comprising: the first network entity measuring channelstate information CSI for the channel between the first and secondnetwork entity; the first network entity selecting a length of the guardinterval based on the CSI; and the first network entity sending thepacket with the guard interval of length selected based on CSI.
 26. Inthe wireless local area network of claim 25, wherein the first networkentity encodes the packet using a capacity enhancing code, theimprovement comprising: the first network appending to a tail end of thepacket an iterative decoding signal extension.
 27. In the wireless localarea network of claim 26, wherein the capacity enhancing code is atleast one of a low density parity check code and a turbo code.
 28. Amethod for transmitting a packet in a wireless local area network WLAN,comprising: determining, at a first network entity, channel stateinformation CSI for a channel between the first and a second networkentity in a WLAN; selecting, at the first network entity, a guardinterval length from among at least two lengths based on the determinedCSI; and sending a packet with the guard interval of the selected lengthfrom the first network entity over the WLAN.
 29. The method of claim 28,further comprising: encoding the packet with a capacity enhancing code,and including in the packet an iterative decoding signal extensiondisposed at a tail end of said packet.
 30. The method of claim 29,wherein the capacity enhancing code comprises one of a low densityparity check code and a turbo code.
 31. The method of claim 28, whereindetermining CSI comprises measuring CSI from a poll message.
 32. Themethod of claim 28 in two iterations, wherein for a first iteration afirst packet comprises a cyclic prefix of a first length and a guardinterval of the second length, and for a second iteration a secondpacket comprises a cyclic prefix of a length different from the firstlength and a guard interval of a length different from the secondlength, wherein the first and second iterations of the method are forthe same first network entity and the same channel.
 33. The method ofclaim 28, wherein selecting a guard interval length comprises selectingfrom among lengths that are integer multiples of 0.4 microseconds. 34.The method of claim 28, wherein selecting a guard interval lengthcomprises selecting from among a first and second length, wherein thesecond length is twice the first length.
 35. A network entity forcommunicating over a wireless local area network WLAN comprising: areceiver for receiving a message from an entity of a wireless local areanetwork WLAN; a memory for storing at least two different guard intervallengths, each associated with a different channel state information CSI;a processor, coupled to the receiver and the memory, for determining CSIfrom the received message, and for selecting from the memory one guardinterval length associated with the determined CSI; a transmitter havingan input coupled to an output of the processor, and an output forcoupling to an antenna, said transmitter output for outputting a packetcomprising a guard interval of the selected length.
 36. The networkentity of claim 35, wherein the transmitter comprises an encoder forencoding the packet with a capacity enhancing code, further wherein thepacket further comprises an iterative decoding signal extension disposedat a tail end of said packet.
 37. The network entity of claim 36,wherein the capacity enhancing code comprises one of a low densityparity check code and a turbo code.
 38. The network entity of claim 35,wherein the message is a poll message and determining CSI comprisesmeasuring CSI from the poll message.
 39. The network entity of claim 35,wherein the processor varies a cyclic prefix of a packet to betransmitted according to the selected guard interval length for thatpacket.
 40. The network entity of claim 35 wherein the at least twodifferent lengths are each multiples of 0.4 microseconds.
 41. Thenetwork entity of claim 35 wherein the at least two different lengthscomprise a first and second length, wherein the second length is twicethe first length.
 42. The network entity of claim 35, wherein thenetwork entity comprises a mobile station.
 43. A device forcommunicating over a wireless local area network WLAN comprising: areceiver for receiving a message from a node of a WLAN; means fordetermining channel state information CSI from the received message;means for determining a guard interval length from at least two lengthsbased on the determined CSI; and means for transmitting a packetcomprising a guard interval of the selected length.
 44. The device ofclaim 43, wherein the means for determining a guard interval lengthcomprise a processor couple to a memory, wherein said memory is forstoring at least two guard interval lengths and at least two values ofCSI, each value of CSI associated with one of the guard intervallengths.
 45. The device of claim 43, wherein the means for transmittingcomprises a transmitter coupled to at least one antenna.
 46. The deviceof claim 43 wherein the received message comprises a poll message, andwherein the means for determining CSI from the received messagecomprises a processor coupled to the receiver for measuring CSI from thepoll message.